An Analytical Alternative to Commercial Window Testing
April 2, 2021
The most obvious way to verify compliance of fenestration products with code requirements for structural strength under wind pressure loading, often based on commercial-grade requirements of the North American Fenestration Standard, is laboratory testing. However, when a specific project calls for non-standard configurations such as larger or smaller sizes, variations in framing configuration or higher wind pressure levels than those of the tested baseline assembly, it can be prohibitively expensive and time-consuming to test all such variations.
Accordingly, the Fenestration & Glazing Industry Alliance offers a standardized process for engineering evaluation of windows and doors, in the form of AAMA 2502-19, Comparative Analysis Procedure for Window and Door Products, an FGIA standard. Approved as a reference standard in the International Building Code, AAMA 2502 sets forth engineering design rules by which commercial products that differ from the test unit can be qualified for code compliance. By providing guidelines for applying loading forces based on the geometry and number of supported panes of the window and door, these rules can also assist manufacturers in the product design effort.
The engineering design rules specified in AAMA 2502 are intended for use in the structural analysis of product framing elements (single span, simply supported beams) subjected to bending under uniform loading, such as that induced by wind. Note that these rules do not apply to the design of infill materials such as glass or panels but can be related to glass loading limitations.
The rules cover 5 elements:
- Load distribution and magnitude. Uniform static loading acting normal to the plane of the window or door.
- Section properties. Area moment of inertia and section modulus for standard and nonstandard or asymmetrical shapes and bending moments.
- Strength. Stress calculations for both tension and compression; calculated stress cannot exceed guidelines in various industry standards for different framing materials. Note that for products with framing spans longer than those of the tested version, calculated deflection is subject to the limitations imposed by AAMA TIR-A11, Maximum Allowable Deflection of Framing Systems for Building Cladding Components at Design Wind Loads, or ASTM1300/CAN-CGSB 12.20, Standard Practice for Determining Load Resistance of Glass in Buildings. If not otherwise defined in the referenced documents, the material yield stress is subject to a safety factor of 1.5 as the basis for allowable stress design.
- Connections. Analyzed for tensile, shear and bearing strength. The calculation of maximum load on each fastener used to anchor the unit to the building is also discussed. Be aware: capacity of fasteners is calculated per AAMA 2501, Voluntary Guideline for Engineering Analysis of Anchorage Systems for Fenestration Products Included in NAFS.
- Deflection. Formulas are provided for figuring deflection and stress limits of simply supported beams, including the calculation of modulus of elasticity, which varies by material, for both symmetrical and unsymmetrical cases. Formulas are also given for calculating the deflection of composite framing sections such as thermally broken framing. Note that calculated frame element deflections cannot exceed those set forth in referenced performance standards (TIR-A11, Maximum Allowable Deflection of Framing Systems for Building Cladding Components at Design Wind Load, and ASTM 1300, Standard Practice for Determining Load Resistance of Glass in Buildings/CAN-CGSB 12.20, Structural Design of Glass for Buildings).
To aid in the implementation of these design rules, AAMA 2502 includes detailed diagrams and associated formulas covering:
- Equivalent concentrated loads (equivalent loads for maximum deflection for rectangular, trapezoidal, and triangular load profiles).
- Distribution of uniform loads (illustrations and calculation formulas for 15 different configurations of framing).
- Section properties (at five locations as identified within a representative framing profile cross-section diagram and associated formulas for first moment, moment of inertia and section modulus).
- Bending moment and deflection formulas for simple beams subjected to uniform (rectangular), trapezoidal and triangular load profiles.
To obtain code compliance for the aforementioned variations applicable to a given commercial project, structural analysis computations encompassing all reasonable failure modes, detailed drawings and test data for windows and doors are typically compiled in the form of an Analysis Report, signed and sealed by a registered professional engineer. AAMA 2502 provides guidance for developing such reports. Engineers and architects can obtain a copy of AAMA 2502-19 by visiting FGIAonline.org/store.